The Chi association has some pretty relaxed ideas about their breed purity.
To wit ... I think if in 1978 a fullblood Chi bull sneezed on a cow (who was bred by a bull of a different breed) before she boarded a trailer heading to a large commercial operation (where she lived a long and productive life), could be registered Chi, along with any of her offspring, and all the other cattle on the trailer with the cow that got sneezed on and all of their offspring could be registered Chi as long as the percentage didn't drop below 1%.
While the above is a bit tongue in cheek, finding a true F1 (fullblood Chi bull on a registered Angus) wikl be very difficult, finding something registered as a Chi - Angus shouldn't be too hard. Heck, many / most club calves have Chi influence.
Good luck.
PS - I believe there is a Chi association in the US ... let google be your friend.